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Significant Figures Show the Uncertainty in Measured Data

Significant Figures Show the Uncertainty in Measured Data. Measured data is written to convey 2 things! the magnitude of the measurement the extent of its reliability. Worker #1 reports a mass of 12 g Worker #2 reports a mass of 12.0142 g. 12 g means 12 ± 1 g

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Significant Figures Show the Uncertainty in Measured Data

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  1. Significant Figures Show the Uncertainty in Measured Data Measured data is written to convey 2 things! • the magnitude of the measurement • the extent of its reliability Worker #1 reports a mass of 12 g Worker #2 reports a mass of 12.0142 g 12 g means 12 ± 1 g 12.0142 g means 12.0142 ±0.0001 g 12 g has 2 significant figures. 12.0142 g has 6 significant figures. 12.0142 g is the more precise (certain, reliable) number The more significant figures a measurement has, the more precise (certain) it is.

  2. Types of Numbers (Data) • Exact • numbers (data) obtained from counting • some conversions (e.g. 2.54 cm = 1 inch, exactly) • Inexact • most measured data Significant figures apply to inexact numbers!

  3. Measured Values: Accuracy vs. Precision accurate and precise accurate but not precise precise but not accurate • Accuracy is how close your measured value is to the right value (can be shown by % error). • Precision is how well you can reproduce your measurement (can be shown by standard deviation). Sig Figs indicate precision.

  4. Making Measurements in the Lab:Recording Thermometer Data to the Correct Number of Significant Figures 23°C 23°C The number of SFs in a measured value is equal to the number of known digits plus one uncertain digit. 22°C 22°C 21°C 21°C you record 21.6°C you record 21.68°C

  5. Making Measurements in the Lab:Recording Volumetric Data to the Correct Number of Significant Figures - Glassware with Graduations Example B Example A 1. If the glassware is marked every 10 mLs, the volume you record should be in mLs. (Example A) 2. If the glassware is marked every 1 mL, the volume you record should be in tenths of mLs. 3. If the glassware is marked every 0.1 mL, the volume you record should be in hundredths of mLs. (Example B) 0 mL 30 mL 20 mL 1 mL 10 mL 30-mL beaker: the volume you write in your lab report should be 13 mL 2 mL Buret marked in 0.1 mL: you record volume as 0.67 mL

  6. Making Measurements in the Lab:Recording Volumetric Data to the Correct Number of Significant Figures - Volumetric Glassware Look on the glassware for written indication of the precision of the volumetric flask or pipet. On this volumetric flask is written 500mL ± 0.2 mL. You would record the volume of the liquid in this flask as 500.0 mL

  7. Making Measurements in the Lab:Recording Masses to the Correct Number of Significant Figures This one is easy: record EVERY number (especially zeros) that appears on the display of the electronic balance. Trailing zeros MUST be recorded.

  8. How to Count Significant Figures • All nonzero digits are significant (1.23 has 3 SFs). • All zeros between nonzero digits are significant (1.003 has 4 SFs). • Leading zeros are NEVER significant (0.01 has 1 SF). • Trailing zeros AFTER THE DECIMAL POINT are significant (0.0780 has 3 SFs). • Trailing zeros that are before the decimal point are ambiguous(100 has 1,2, or 3 SFs). If possible, use scientific notation to eliminate the ambiguity.

  9. Scientific Notation • An unambiguous way to show the number of significant figures (SFs) in your data • Numbers are written as the product of a number greater than or equal to 1 and less than 10 and a power of 10 Measurement in scientific notation #SFs 1.86282 x 105 mi/s 5.1900 x 10-3 m 5.121 x 103 g 6 5 4 186282 mi/s 0.0051900 m 512.1 x 101 g

  10. Maintain the Correct Number of SFs Multiplying or Dividing Measured Data • The answer contains the same number of SFs as the measurement with the fewest SFs. 25.2 x 6.1 = 153.72 (on my calculator) = 1.5 x 102 (correct answer) 25.2 ------------ = 7.3122535 (on my calculator) 3.44627 = 7.31 (correct answer) (6.626 x 10-34)(3 x 108) ------------------------------- = 3.06759 x 10-2 (on my calculator) 6.48 x 10-24 = 0.03 (correct answer)

  11. Maintain the Correct Number of SFs When Performing Calculations Involving Measured Data • Addition/Subtraction: The answer contains the same number of digits to the right of the decimal as that of the measurement with the fewest number of decimal places. 33.14159 - 33.04 0.10159 0.10 (correct answer) 2 SFs 3.14159 + 25.2 28.34159 28.3 (correct answer) 3 SFs • Calculators do NOT know these rules. It’s up to you to apply them!

  12. Maintaining the Correct Number of SFs When Working with Common Logarithms • Log x = y or 10y = x The number of decimal places in y is the number of SFs in x. Example 1. Log x = 2.33 (2 decimal places) x = 213.796209 x = 2.1 x 102(2 SFs) Example 2. x = 561.3 (4 SFs) Log 561.3 = 2.749195042 Log 561.3 =2.7492 (4 decimal places) • Calculators do NOT know these rules. It’s up to you to apply them!

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